We have shown tumor necrosis factor (TNF) to be a multi-functional regulator of normal postnatal mammary gland development. Our data suggests that TNF, acting through the p55 TNF receptor, plays an active stimulatory role in the rapid proliferation that occurs during pregnancy, and in the proliferation and branching morphogenesis that occur during puberty. Significantly, branching morphogenesis of the mammary ductal epithelium is inhibited in TNF null mice, demonstrating the physiological relevance of this cytokine in the mammary gland. Surprisingly, in spite of numerous studies demonstrating that TNF inhibits the growth or induces apoptosis of breast cancer cell lines, we found that TNF stimulates the growth of mammary tumor epithelial cells in primary culture, suggesting that TNF therapy may be contra-indicated in breast cancer. The stimulatory effect of TNF on proliferation of normal _ epithelial cells (MEC) is accompanied by an increase in DNA-binding activity of the NFkB1/p50 homodimer. Moreover, in newly isolated tumor MEC in which p50 protein and NFkB1/p50 homodimeric-DNA binding are absent, TNF does not affect cell growth; however, upon p50 expression in the tumor cells, TNF induction of proliferation is observed. This suggests that NFkB1/p50 is required for TNF stimulation of growth of both normal and malignant MEC. In Aim 1, the mechanism by which NFkBI/p50 expression and activity are regulated by TNF in normal and malignant MEC will be investigated. Aim 2 will determine if there is an absolute requirement for the NFkB1/p50 protein in TNF-induced proliferation of MEC, or whether another NFkB/rel protein can functionally compensate for p50. Aim 3 will determine the sensitivity of the mammary gland from TNF null mice to carcinogenesis. Since TNF stimulates the growth of both normal and malignant MEC in primary culture, we propose that it may stimulate, rather than inhibit manmaary tumor growth. Thus, as part of this aim, the efficacy of anti-TNF therapy vs local TNF therapy against mammary tumors will be compared.